An alkylene oxide adduct has heretofore generally been produced by a batch-type reaction method. The batch-type reaction method is a method for producing an alkylene oxide adduct by charging an organic compound having an active hydrogen atom(s) as a starting material and a catalyst in an autoclave, and after injecting an alkylene oxide, reacting these materials under pressure at a predetermined temperature (for example, see Patent Documents 1 to 3). However, in the method for producing the alkylene oxide adduct by such a batch-type reaction method involves the problems that 1) the alkylene oxide adduct cannot be produced continuously, 2) there is a risk of explosion since the addition reaction under high pressure is carried out in a large-sized apparatus, 3) a temperature or a pressure at the time of the addition reaction is difficultly controlled since the reaction is carried out by the gas-liquid reaction, so that quality of the product is lowered that coloring is generated and the molecular weight distribution is broadened, etc., and 4) the producing apparatus is expensive, etc. As an improving method of such a batch-type reaction method, a continuous reaction method using a tube reactor (for example, see Patent Documents 4 and 5) has been proposed, but the method for producing the alkylene oxide adduct by such a conventional continuous reaction method involves the problems that 1) a temperature or a pressure at the time of the addition reaction is still difficultly controlled, so that quality of the product is lowered that coloring is generated and the molecular weight distribution is broadened, etc., and 2) mass production is difficult, etc.